Current foundry practice utilizes induction furnaces to melt and hold metals; consequently, greater demands than ever have been made upon both the induction furnace manufacturer, as well as the crucible producer, to provide equipment which will melt at faster rates. To help meet these demands, newer, higher powered high frequency furnaces are now available having a frequency range of 1000 to 3000 Hertz. These operating conditions have a devastating effect on crucible compositions which are unable to repeatedly withstand such severe treatment. That is, some crucible compositions might tend to have too high electrical conductivity, thereby causing heating at such a rapid rate that the crucibles develop isolated hot spots, blister, shatter, or become just too hot to meet the operating requirements of a given foundry. Too high thermal conductivity can result in excessive metal temperature, heat loss during the pouring operation, and can also cause the crucible to become too hot to meet the operating requirements of a given foundry.
On the other hand, crucibles having too low electrical conductivity, also have a low susceptibility to the induction field, therefore tend to crack as a result of thermal gradients in the crucible. These are caused by the combination of the molten metal level and induction field, promoting uneven heating between the midwall of the crucible and its top and bottom sections. Furthermore, crucible compositions with too low susceptibility to the electrical field heat up at too slow a rate, and therefore contribute less to the melt rate of the charge.
It has been found that the crucible compositions disclosed below, and which form the basis for this invention, have the optimum levels of higher electrical resistivity, lower susceptibility to the induction field, lower thermal conductivity, and lower thermal expansion. This combination reduces the likelihood of the crucible failing due to thermal shock, blistering, or overheating while at the same time maintaining the desired melt rate with minimum heat losses during the pouring operation.
Following modern practice, particulate raw materials are dry blended, then, in the case of a clay-bonded crucible, such as this invention deals with, are mixed with 7 to 20% by weight water. The "soft mud" mix is then formed or molded into crucibles by any one of several acceptable means, such as mechanical pressing, jiggering, spinning, hand ramming or isostatic pressing. All these processes are currently in use by the crucible manufacturing industry, and are considered to be conventional ceramic processes.
Following forming, the crucibles are then dried, and may be covered with any one of a wide variety of glaze coatings to prevent oxidation of the graphite during initial firing, or when subsequently used by the consumer. However, one of the unexpected advantages of the crucible of the instant invention is that its useful life can be greatly enhanced by the controlled oxidation of the internal crucible surface during manufacturing. This is accomplished by eliminating the conventional glaze on the interior of the crucible, prior to firing, and applying a coating which will retard deep oxidation, yet at the same time permit a thin film of surface oxidation to occur. This is accomplished by coating the dried crucible interior with an engobe which contains about 30 to 50% by weight silicon metal, 20 to 60% by weight of ball clay and 0 to 50% clay-graphite random fines collected from the mixing operation of the main body. The engobe coating is applied to a thickness of between 0.005" and 0.040". Upon firing, there results a slightly oxidized layer consisting mostly of silica, on the crucible interior, which becomes an integral part of the crucible. The depth of the oxidized layer can range from 0.06" to 0.25".
More particularly, the oxidized layer induced as described above is composed of the same raw materials as the main body, with the exception that the carbon from the graphite has been burned out through oxidation, leaving a porous carbon-free structure which serves as an electrical insulator between the metal and the main crucible body composition, and which also serves to further reduce the thermal conductivity between the two.
Whether glazed inside and out, or whether glazed only on the outside, with an oxide-producing engobe on the inside, the crucibles are fired, following drying, to a temperature in the range of 1250.degree. to 1550.degree. C. The preferred firing temperature is 1400.degree. C., or about cone 14, in an oxidizing atmosphere. Regardless, any composition as disclosed hereinafter, may be fired, following minor trial and error, at a temperature sufficient to partially vitrify the clay so as to form the bond necessary to obtain sufficient mechanical strength to hold the crucible together. At the same time, firing is on a fairly rapid schedule so as to minimize crystallization or devitrification of the fused silica. At 1400.degree. C., the preferred firing schedule would be one completed in about twenty-four hours, held at peak temperature briefly, then permitted to cool back down to room temperature in about sixteen hours.